A control device of an induction electric motor that is the power source of an electric vehicle converts a run command, ordering output from the induction electric motor, into a control signal that is output to an inverter main circuit. The inverter main circuit converts inputted DC electric power into variable voltage-variable frequency AC electric power and drives the induction electric motor.
The electric power supplied to the electric motor, in addition to being converted into shaft output of the electric motor, is converted into iron loss and copper loss, which are internal losses of the electric motor. These iron and copper losses become thermal energy and cause a rise of temperature of the electric motor.
Control devices for electric motors (also referred to below simply as “motors”) often lack over-temperature protection for the motor due to omission of a temperature sensor due to difficulty of placement of the temperature sensor in the electric motors, as well as high cost and the like. When over-temperature protection is provided, protective sensing and the like occurs when the motor current effective value exceeds a threshold for at least a fixed time period, or when an estimated motor temperature or a corresponding equivalent loss exceeds a threshold value. For example, according to Patent Literature 1, a main electric motor is determined to have a temperature state exceeding a standard state based on a motor current effective value.
According to the electric motor control method of Patent Literature 2, temperature of the other windings is estimated based on temperature detected by a temperature sensor arranged at a position for detection of temperature of one winding among windings supplying three-phase AC current.
Temperature is inferred from resistance value by use of the fact that resistance value of the winding is proportional to its temperature. The electric motor protective device of Patent Literature 3 uses voltage and current value of each frequency as input data for calculation of impedance, and calculates temperatures of the electric motor stator and windings of rotor based on the resistance component of impedance. According to the motor heating-protection device of Patent Literature 4, motor applied voltage, rotation rate and motor current are used to calculate winding resistance, and a winding temperature θ is calculated based on the winding resistance. Moreover, according to the temperature-change sensing method for a rotary motor of Patent Literature 5, each value of coil current, voltage across the coil terminals and rotor rotation rate is used to calculate a coil line-to-line resistance value.